Chart plotters: The best is yet to come

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Electronic chart plotters have become far more capable in recent years. Yet there are new developments available, or on the horizon, that will be extremely valuable.

Electronic chart plotters have become far more capable in recent years. Yet there are new developments available, or on the horizon, that will be extremely valuable.

Today, you can get a wide variety of chart plotters: stand-alone, integrated with a GPS receiver, operating with a radar or depth finder screen, or based on a laptop computer. The

number of combinations and systems is almost bewildering. Nearly any chart plotter can be an excellent addition to a GPS receiver, and many have larger screens than GPS receivers, giving a better view. The foreseeable changes will make them more capable than ever.

A chart plotter may have an internal GPS receiver or use a separate one, and the receiver should be WAASor DGPS enhanced. Both the Wide Area Augmentation System and Differential GPS provide significant improvements in the accuracy of GPS positions, speed and direction of travel.

Chart plotters allow electronic trip planning, including setting route waypoints. This establishes courses and distances between waypoints. And you can move the waypoints to a GPS receiver, individually or as part of a route. Although planning long legs of a trip is more awkward with a chart plotter than with a paper chart, planning with a plotter is easy in confined waters with short trip legs and many waypoints. You can designate and save a waypoint merely by moving the pointer to the right place and clicking on it.

Keeping charts up to date is vital, but it has been costly and time-consuming, which has kept many boaters using old chart data long after there have been changes to the charts. The Coast Guard installs or moves aids to navigation, shoals appear, and new buildings, antennas and tanks are built. Old charts don’t show any of this newer data.

Now some of the chart plotters and computer software can download up-to-date charts at no cost from the National Oceanic and Atmospheric Administration or from the software supplier. This is a big advantage, in my opinion. When deciding on a chart plotter, be sure to investigate the method and cost of updating charts.


Charts are the essence of chart plotters. Many good plotters use raster charts, which are scanned from paper charts. It is relatively inexpensive to scan a chart into the raster format, and they can be updated quickly. When used on sophisticated plotters or computers, they offer advantages over paper charts. A chart plotter allows you to plan, designate waypoints, zoom in to see more detail, and zoom out to see more of the route. And manufacturers have added many features, such as radar overlays, aerial photographs, extra surveys for small areas, and three-dimensional representations of the bottom.

Bear in mind that zooming in does nothing to improve the accuracy of raster charts; in fact, it is misleading. When you zoom in a raster chart, the writing gets larger. When you zoom out, the writing gets smaller and the screen becomes cluttered with information packed close together. These are inevitable drawbacks of raster charts.

NOAA has scanned all of its paper charts in raster format, called RNCs, and offers them for free download at There also is a free viewer available.

Not all chart plotters and software programs that use raster charts are equipped to obtain the free downloads; many plotters must be updated by purchasing chart cards on a periodic basis. NOAA has a list of manufacturers that produce systems that use RNCs at the above Web site.

As we move to enhanced presentation of various independent inputs, the International Hydrographic Office has developed the Electronic Navigational Chart, or ENC, international exchange format S-57 to comply with the IHO ENC chart specifications. The leading chart-making nations are offering ENCs in the S-57 format, which offer greatly expanded information to the navigator. ENCs are designed to be used with an ECDIS, an internationally accepted standard for Electronic Chart Display and Information Systems.

NOAA first developed ENCs for the major ports and has continued to add other areas. ENCs are in vector format and include far more information than can be shown on a paper (or raster) chart. NOAA ENCs are actually a database, including data from the original surveys. The surveys are done at higher accuracy than can be shown on a paper chart. They use accurate channel limit data from the Army Corps of Engineers and aids to navigation positions from the Coast Guard. NOAA modifies the data to include Notice to Mariners and other critical information on a frequent basis. In addition, the database nature of an ENC makes it possible for a system using it to compare the charted depth with the ship’s position.

NOAA now has produced 610 ENCs and has made them available for free download. Begin at nauti and click on “electronic navigational charts.” NOAA lists systems and viewers using ENCs at

The charts underlying either raster charts or ENCs are always a limiting factor. Just because they look pretty on the screen doesn’t mean they are accurate. More than half of the soundings on today’s NOAA charts are based on hydrographic surveys taken prior to 1940. It takes a very long time to resurvey everything, and NOAA does the survey work for completely new charts on about a 50-year cycle. Of course, it concentrates on deep-draft channels and routes to give the highest safety to large ships, but we who operate boats often do so in water too shallow for a ship — even a small ship. It seems obvious that such areas will get the least attention.

In using a chart plotter, I have had to go over what appeared to be dry land to follow the channel at our local inlet in North Carolina. On the other hand, your chart plotter may show the boat in good water that isn’t really good. Last year a 38-foot sportfisherman was approaching Beaufort, N.C., from the Intracoastal Waterway at Adams Creek, and headed down Gallant’s Channel. The operator was having some trouble seeing the buoys, but lo and behold the plotter showed the boat between the two lines of buoys. What, me worry? The boat grounded at 28 knots, with severe injuries to people and boat. The Coast Guard had pulled the buoys months earlier, when the channel shoaled too much to allow safe navigation.

In addition, we now use DGPS and WAAS-assisted GPS, which typically give higher accuracy than was available when the NOS performed the chart survey. So WAAS and DGPS may well be “accurate” with respect to their coordinate system, but the chart may not “fit” that grid for a variety of reasons. In other words, the deep water isn’t where the DGPS shows the boat to be. Thus, there are many places where a chart on the screen doesn’t match the channel.

You can overcome these problems in the same way for any electronic position-fixing system: set waypoints at accurate positions marking key points in the channel. For example, set a waypoint at the intersection of two range lines that mark a turn in a channel. Going very slowly, maneuver the boat to this accurate visual position and save a waypoint. It is important to go slowly to avoid the averaging lag of your particular receiver. That waypoint, saved with your WAAS or DGPS receiver, is more accurate than one you can obtain by measuring on any type of chart.

We also save waypoints about 100 yards offshore of an entrance channel midline, and others at turns. Those waypoints are in good water by the depth finder and align with the channel markers and buoys visually. They aren’t as accurate as those at range intersections, but they are a great deal more accurate than those you can plot from a chart. They are especially valuable for shallow inlets that don’t show buoys on the charts.

The Army Corps of Engineers Wilmington, N.C., District has pioneered the effort to determine waypoints for each channel leg intersection during its regular sounding surveys. They show these intersection positions and those of buoys, daybeacons and lights in latitude and longitude for convenient entry into GPS receivers. Previously they showed State Plane coordinates only, which were awkward to convert to latitude and longitude. With most of today’s systems, you must copy the waypoints manually. I expect it will become easier to enter this data in the future, as it is now easy to update a computer program over the Internet.


Both vector and raster systems can overlay radar to show the radar data for shorelines, ships, aids to navigation and many other targets. It is especially useful to show ships and boats against a nautical chart. Some systems can even show the Automatic Identification System data that ships broadcast.

However, radar doesn’t consistently show an image that matches a chart. Many beaches slope gradually and show radar echoes inshore of the actual shoreline. And radar always shows a point or an island wider than it is, due to the radar beamwidth. Longer radar antennas are better than short ones in this respect, but they all show the effect. Radar cannot show land features over the crest of a hill, since radar uses line-of-sight radio frequencies. It takes some experience to match a radar picture with a chart, but radar overlaid on a chart plotter speeds up the learning process.

The path ahead

As useful as chart plotters are, a series of automatic, accurate position plots is only one part of navigation. A navigator must plan a safe route and cross-check every piece of information with other data. Thus, a navigator needs to compare fixes (of every type) with the depth and the proposed course. Normally this involves plotting a DR (dead-reckoning) position and comparing it with fixes to reveal set and drift of the current, errors and necessary changes. It is most important for a navigator to examine the course ahead of the ship to see if the present track will cross any hazards. As I mentioned, it is possible to program a chart plotter or computer using an ENC to look ahead of the ship to identify hazards by distance, type and time to arrive. This is the age-old problem for navigators and goes far beyond the first-generation equipment that plotted vessel position continuously.

Many ships have stranded when the navigator failed to examine the path ahead for hazards after changing from the planned course to avoid a ship or for another reason. There are many notorious examples of this: the 1995 Star Princess grounding on Poundstone Rock in Alaska, the 1992 Queen Elizabeth 2 grounding in Buzzard’s Bay, Mass., and, of course, the 1989 Exxon Valdez grounding on Bligh Reef in Alaska. These and many other groundings occurred when the officer of the watch concentrated on one thing, such as an approaching ship, and overlooked hazards ahead on an altered course.

In order for a chart plotter to provide the best input for warnings of an impending grounding, it is necessary to adapt the software to account for the ship’s characteristics, including turning radius and stopping distance. Obviously, the system must alert the navigator well before it is too late to take effective action. This is far beyond many of today’s systems, which will merely show what was hit after the ship stops. But some systems today using ENCs offer an early version of this feature.

This isn’t a new problem, and it’s been addressed in the past. The Iotron Company developed an add-on to radar back in the 1970s that did exactly this. It was called Digiplot and was the brainchild of Jack Herther. Digiplot had many advanced features, such as automatic acquisition of radar targets, fully automatic tracking, and consolidation of 3-cm and 10-cm radar information. It was limited by computing power and memory costs.

With today’s ENCs, a system based on this technology could make a significant improvement in ship safety. The great increases in computing power and data storage make this type of system far more affordable than before.

It took about three decades to go from manually plotting radar targets, to determine risk of collision and predict the effect of proposed maneuvers, to the Automatic Radar Plotting Aid that is used aboard ships. The job of developingand implementing a system to predict groundings from chart plotter and radar data is more complex but within reach. I see it as a problem that requires international cooperation and standards to be fully effective.

Position inputs

With all the capabilities of chart plotters, there remain some disadvantages. Most have only one position input. It is important to have two independent, continuous sources of position data (actually position, velocity and time) in order to detect errors in the signal system or in the receiver. Ideally, the position data inputs should be as independent as possible to avoid problems due to man-made or natural interference, the receiver or the transmitted signals.

I have been aboard large ships that had dual position feeds to the chart plotter: identical GPS-WAAS receivers. This setup is weak, since interference, time-dependent software errors or signal errors affect both GPS receivers simultaneously. It is a little better to have two GPS receivers from different manufacturers. In time, it may be practical to have a GLONASS or a Galileo receiver as the second position input. This would be better, yet electromagnetic interference in the GPS band will affect the other two satellite-based systems simultaneously.

Loran-C, where available, provides an excellent independent system. It is subject to interference but in a completely different frequency band. It isn’t as accurate as WAAS or DGPS, and like them it can have signal errors. But today it is the best complementary system to GPS, in my opinion. Loran has excellent short-term stability (a day or less) and can be calibrated by a GPS receiver. Then the two inputs will show two little ship icons, very close together, crawling across the chart. If they stray apart, voila! One of them has a problem. The navigator can earn his or her keep by finding out which one is wrong.

Depth finder inputs

There are many chart plotters that show depth finder information on a split screen. This allows correlation of fix positions with depth, an essential cross-check. Now there is an even more advanced method of combining the two systems. Several manufacturers are offering sophisticated software to connect depth finders with chart plotters and augmented GPS receivers. This allows a boat operator to criss-cross a specific area on the bottom, such as a ledge or a wreck, and gather data to build a 3-D model of the bottom. The results are truly impressive. Some of these systems also can determine the character of the bottom: mud, sand, pebbles or gravel. Commercial fishermen are the primary users of these systems now.

I’m reminded of an old tale in Harold B. Clifford’s “Charlie York: Maine Coast Fisherman.” Capt. Sol Hayes of Gloucester could identify spots in the Gulf of Maine by touching, tasting and biting on pebbles or sand that came up attached to the lead. He liked to have a few drinks, too. Once, one of his crewmembers brought some dirt from a vegetable garden and hid it in his gear. After the boat got under way for the halibut grounds on Georges Bank, the skipper went to his bunk and continued drinking. He asked the mate to arm the lead and pick up something from the bottom from time to time, then he’d tell him where to steer. The cook packed a bit of the shoreside dirt in the lead, sloshed it overboard, shook the skipper awake, and handed it to him. Sol tasted it, whimpered, then began to cry. He said, “Boys, I’ve been skipper for 18 year, and this is the first time I’ve ever been lost in my life. We’re right in the middle of somebody’s cabbage patch.”

Fishermen need to know the character of the bottom more than ever. Ship officers need information about the vessel’s position, the hazards ahead, and the details of the channel. And boat operators need detailed information from which to plan their navigation and avoid hazards. Chart plotters are making tremendous advances in all of these fields.

Capt. Bill Brogdon is the author of “Boat Navigation for the Rest of Us” (International Marine/McGraw Hill). A lifelong boater, he is a retired Coast Guard captain who commanded three ships.